This application is a 371 of international application PCT/US09/58131 filed Sep. 24, 2009, which claims the benefit of provisional application 61/099,855 filed Sep. 24, 2008.
In 2005, the EPA issued the Clean Air Mercury Rule to cap and reduce mercury emissions from coal-fired power plants. This rule, combined with the EPA's Clean Air Interstate Rule (CAIR) or other rules, may require significant reduction in mercury emissions from coal-fired power plants in the U.S. as early as 2010.
Significant coal resources exist around the world that have the potential to satisfy much of the world's energy needs for a long period of time. The U.S. has large amounts of low-sulfur coal sources, e.g. Powder River basin coal in Wyoming and Montana, but such sources contain non-negligible amounts of mercury in both the elemental and oxidized forms. Thus, some type of mercury emission mediation technology is necessary in order for coal-fired energy plants to utilize such sources of coal without substantial mercury emissions.
The Department of Energy has presented information from several studies that indicate mercury emissions during combustion of coal fuels can be lowered by treatment of the coal fuel stocks with low levels of bromine.
Brines that are produced in several areas of the world contain substantial quantities of bromide salts, such as sodium bromide. Bromine can be recovered from such brines by treatment with chlorine to oxidize the bromide to bromine. Processes for electrolytic conversion of bromide to bromine are also known; but electrolytic conversion is an expensive alternative to the aforedescribed process. Catalytic oxidation of bromide to bromine by use of oxygen or air mixtures has been reported; but no successful, economic, commercial operation is in place today.
It is known to remove hazardous gaseous components from a gaseous effluent by dispersing a fine particulate sorbent evenly in the effluent to contact and capture, in flight, the targeted gaseous component followed by mechanically removal of the sorbent with its adsorbate from the effluent vapor by electrostatic precipitators (ESP), fabric filters (FF), or wet scrubbers. A highly efficacious sorbent is powdered activated carbon (PAC). The PAC can be used with or without modification. Modified PACs are claimed to enhance capture of the target hazardous substance by enhancing adsorption efficiency. PAC modification is exemplified by U.S. Pat. No. 4,427,630; U.S. Pat. No. 5,179,058; U.S. Pat. No. 6,514,907; U.S. Pat. No. 6,953,494; US 2001/0002387; US 2006/0051270; and US 2007/0234902.
Thermal stability can be problematic with PACs and other sorbents; e.g., when a PAC is used in the treatment of warm or hot gaseous effluents or when packaged or collected in bulk amounts, self-ignition can result from unmitigated oxidation of the PAC and can lead to its smoldering or burning. Bulk PAC can be encountered, e.g., when the PAC is packaged, such as in super-sacks or when formed as a filter cake in a FF unit or collected in silos or hoppers associated with an ESP, etc. Self-ignition can be exacerbated by the PAC being warm or hot as could be the case when treating coal-fired boiler effluents. If oxygen (air) is not denied to the oxidation site or if the site is not cooled, the heat from the initial oxidation can propagate until the PAC smolders or ignites. Such an ignition can be catastrophic. Utility plants are especially sensitive about self-ignition as smoldering or fire within the effluent line can cause plant shut-down with widespread consequences to customers.
Given the foregoing, it would be commercially beneficial to have new processes for minimizing mercury emissions from coal and other fuel stocks. Additionally, it would be advantageous to have PACs and other sorbents with improved thermally stability.